Fiber-reinforced composite materials in which carbon fibers or aramid fibers are used as reinforcement fibers, are widely used, taking advantage of their high specific strengths and specific elastic moduli, for structural materials for aircraft, automobiles, etc.; and for purposes such as sports/general industrial applications including tennis rackets, golf shafts, and bicycles.
In most golf clubs in recent years, the heads are made of metal, and moreover, head sizes have increased. The cause increased vibrations transmitted to hands and an increased sense of rigidness during ball hitting. Consequently, there has been an increasing demand for improvements in vibration damping properties and in the feeling of ball hitting.
Patent Document 1 discloses a golf shaft in which a vibration damping layer formed of a metal fiber braid is provided at least in a portion in the longitudinal direction of a shaft formed of fiber-reinforced resin. However, in this case, while vibration damping properties are improved, the shaft mass increases because they comprise metal fibers having higher specific weight than that of carbon fibers, which are usually used for golf shafts.
Furthermore, Patent Document 2 discloses a golf shaft in which one or more layers of polyester are provided between fiber-reinforced resin layers or on the inner surface of the innermost fiber-reinforced resin layer. In this case, while vibration damping properties are improved, the flexural strength and the torsional strength of the shaft decrease because the modulus of such polyester is lower than that of epoxy resin, which is usually used as matrix resin for fiber-reinforced resin for golf shafts.
As for tennis rackets as well, there are demands for weight reduction, better resiliency, and improved operability. Thus positive use is made of carbon fiber composite materials in which epoxy resin is used as matrix. A shock applied when a ball is hit using a racket is transmitted as vibrations to a human body. It is known that if these vibrations are repeatedly received, fatigue accumulates in the elbow, thus causing a so-called tennis elbow, in which pain develops in the elbow. It has been agreed that such vibrations are preferably reduced.
Patent Document 3 discloses a tennis racket made of a fiber-reinforced composite material in which an epoxy resin composition that contains a specific epoxy resin component, epoxy-incompatible rubber particles and polyvinyl formal, is used as matrix resin. In this case, some rubber particles which are incompatible with epoxy resin enter the interior of reinforcing fiber bundles and the other rubber particles are filtered, with the result that a larger quantity of rubber component exists on the surface of the prepreg than in its interior. Consequently, subsequent to lamination, a large quantity of rubber component can exist between prepreg layers. Therefore, it is possible to provide a racket which has higher vibration damping properties and excels in hitting feeling, as compared to a case where minute rubber particles which are soluble in epoxy resin are used to allow the rubber component to be distributed uniformly. However, since rubber particles having lower modulus than an epoxy resin enter the interior of reinforcing fiber bundles, the matrix resin's modulus decreases, resulting in a decrease in the racket rigidity and strength.
Meanwhile, Patent Document 4 exemplifies a prepreg which is characterized in that the entry of particles into reinforcing fiber bundles is restricted, and that a large number of particles are present on the surface of the prepreg. In this case, polyamide 12 particles are applied. Although shock resistance and fatigue properties improve, vibration damping properties near room temperature are not sufficient.
Furthermore, Patent Document 5 exemplifies a fishing rod in which vibration damping properties are enhanced for the purpose of transmitting slight fish bites with high sensitivity. In this document, a urethane elastomer sheet is placed between fiber reinforced resin layers. This elastomer sheet greatly improves vibration damping properties, but significantly reduces the rigidity and the strength of the fiber reinforced material.